Murine Go T-cells from young (2-4 mo.) and old (24-27 mo.) animals do not express detectable levels of high or low affinity IL-2 receptors. After 40 hrs. of incubation with either PMA or Con A together with recombinant IL-2, the number and affinity of IL-2 receptors is indistinguishable by age. However, the number of cells surviving the incubation period is remarkably lower in cultures containing T-cells from the older mice. Similarly prepared G0 T cells when incubated with Con A for 8 or 16 hrs. expressed high affinity IL-2 receptors in an age-related fashion. Cells derived from old animals failed to develop high affinity IL- 2 receptors as detected by radioligand binding assays. Cells derived from young animals expressed detectable levels of high affinity IL-2 receptors 8 hrs. after incubation with Con A and by 16 hrs. the number of these receptors increased significantly. Similar studies performed with phorbol diesters in place of Con A showed no age-associated difference in the expression of high or low affinity IL-2 receptors as detected by radioligand binding. The high affinity IL-2 receptors induced by phorbol diester fail to deliver a proliferative signal under physiological conditions. Extremely high levels of r-IL-2 are required to induce minimal proliferative responses in phorbol diester induced cells irrespective of the age of the mice. Estimation of the frequency of G0 T cells able to respond to Con A by expressing functional high affinity IL-2 receptor showed a significant decrease in the population of cells derived from old mice.